1. Field
The present invention relates generally to data communication, and more specifically to techniques for transmitting data using a number of diversity transmission modes in MIMO OFDM systems.
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, packet data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users either sequentially or simultaneously. This is achieved by sharing the available system resources, which may be quantified by the total available operating bandwidth and transmit power.
A multiple-access system may include a number of access points (or base stations) that communicate with a number of user terminals. Each access point may be equipped with one or multiple antennas for transmitting and receiving data. Similarly, each terminal may be equipped with one or multiple antennas.
The transmission between a given access point and a given terminal may be characterized by the number of antennas used for data transmission and reception. In particular, the access point and terminal pair may be viewed as (1) a multiple-input multiple-output (MIMO) system if multiple (NT) transmit antennas and multiple (NR) receive antennas are employed for data transmission, (2) a multiple-input single-output (MISO) system if multiple transmit antennas and a single receive antenna are employed, (3) a single-input multiple-output (SIMO) system if a single transmit antenna and multiple receive antennas are employed, or (4) a single-input single-output (SISO) system if a single transmit antenna and a single receive antenna are employed.
For a MIMO system, a MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, with NS≦min {NT, NR}. Each of the NS independent channels is also referred to as a spatial subchannel of the MIMO channel and corresponds to a dimension. The MIMO system can provide improved performance (e.g., increased transmission capacity and/or greater reliability) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized. For a MISO system, only one spatial subchannel is available for data transmission. However, the multiple transmit antennas may be used to transmit data in a manner to improve the likelihood of correct reception by the receiver.
The spatial subchannels of a wideband system may encounter different channel conditions due to various factors such as fading and multipath. Each spatial subchannel may thus experience frequency selective fading, which is characterized by different channel gains at different frequencies of the overall system bandwidth. It is well known that frequency selective fading causes inter-symbol interference (ISI), which is a phenomenon whereby each symbol in a received signal acts as distortion to subsequent symbols in the received signal. The ISI distortion degrades performance by impacting the ability to correctly detect the received symbols.
To combat frequency selective fading, orthogonal frequency division multiplexing (OFDM) may be used to effectively partition the overall system bandwidth into a number of (NF) subbands, which may also be referred to as OFDM subbands, frequency bins, or frequency sub-channels. Each subband is associated with a respective subcarrier upon which data may be modulated. For each time interval that may be dependent on the bandwidth of one subband, a modulation symbol may be transmitted on each of the NF subbands.
For a multiple-access system, a given access point may communicate with terminals having different number of antennas at different times. Moreover, the characteristics of the communication channels between the access point and the terminals typically vary from terminal to terminal and may further vary over time, especially for mobile terminals. Different transmission schemes may then be needed for different terminals depending on their capabilities and requirements.
There is therefore a need in the art for techniques for transmitting data using a number of diversity transmission modes depending on the capability of the receiver device and the channel conditions.